The invention relates generally to measurement of characteristics of a non-Newtonian fluid and more specifically to a method and an arrangement for measuring viscoelasticity of a non-Newtonian fluid, particularly blood.
Blood viscosity and viscoelasticity are missing or at least overlooked factors in health care. One of the leading causes of death in the western world is formation of blood clot (thrombus) in a blood vessel. The more viscous the human blood is, the greater the risk is of developing vessel damage that can lead to a heart attack or a stroke. Mounting evidence indicates that the blood viscosity is elevated among patients with essential hypertension and that blood stickiness is an early development of atherosclerosis. Viscoelasticity of blood is associated with pathologies such as peripheral vascular disease and diabetes.
Blood viscosity and viscoelasticity are low at high shear rates. However, low shear rates arise at sites of atherogenesis, thrombogenesis, and ischaemia, promoting local hyperviscosity. This suggests that abnormalities in haemorheological factors be involved in the complication of hypertension and determination of blood pressure.
Blood is a suspension of mostly elastic particles, i.e. cells, in a Newtonian fluid, i.e. plasma, which results in non-Newtonian characteristics in that whole blood viscosity and viscoelasticity are dependent upon the shear rate the blood experiences.
U.S. Pat. No. 4,643,021 discloses a method and apparatus for measuring the viscosity of biological fluids, e.g. blood, by means of a rotating cylinder immersed in the fluid in a tube and on the basis of a torque applied to the cylinder and the speed of rotation of the cylinder. The cylinder is rotated by a rotating electromagnetic field produced by an induction winding that coaxially surrounds the tube and the cylinder and that is powered with current at an adjustable intensity.
The method disclosed in the above U.S. Pat. No. 4,643,021, utilizing a rotating cylinder, is not sensitive enough for measuring deformation of red blood cells and, consequently, it is not possible to measure viscoelasticity of e.g. blood by means of that method.
The object of the invention is to bring about a simple and inexpensive method and arrangement enabling measurement of viscosity and viscoelasticity of non-Newtonian fluids, particularly blood.
This is attained by the method according to the invention in that the fluid is received in a non-magnetic container containing a magnetic body, a winding is provided around the container, input AC currents of different amplitudes are supplied to the winding to generate magnetic fields of different strengths in the container to cause the magnetic body to oscillate at different shear rates in the fluid, amplitude and/or phase of the oscillations of the magnetic body are measured, and the shear rates are determined by measuring differences between the amplitude and/or phase of the oscillations of the magnetic body and the amplitude and phase of the input AC currents, the amplitude difference at a specific shear rate being proportional to the viscosity of the fluid, and the phase difference at a specific shear rate being proportional to the viscoelasticity of the fluid.
The object is also attained by means of the arrangement according to the invention, comprising a non-magnetic container for receiving the fluid, a magnetic body being contained in the container, a winding being provided around the container, an AC current supplying device being connected to the winding for supplying input AC current of different amplitudes thereto to generate magnetic fields of different strengths in the container to cause the magnetic body to oscillate at different shear rates in the fluid, a measuring device being provided for measuring amplitude and/or phase of the oscillations of the magnetic body, and a shear rate determining device being provided for determining the shear rates by measuring differences between the amplitude and/or phase of the oscillations of the magnetic body and the amplitude and/or phase of the input AC currents, the amplitude difference at a specific shear rate being proportional to the viscosity of the fluid, and the phase difference at a specific shear rate being proportional to the viscoelasticity of the fluid.
By means of the method and the arrangement according to the invention, e.g. clinical measurements of blood viscoelasticity will be possible, simply using a single blood sample.